Surface-Functionalized Silicon Nanoparticles as Contrast Agents for Photoacoustic Microscopy Imaging

We demonstrate high contrast optical-resolution photoacoustic microscopy (OR-PAM) imaging by using biocompatible, indirect band gap silicon nanoparticles (SiNPs) as contrast agents. The SiNPs were synthesized by high-energy ball milling of crystalline silicon wafers, followed by hydrosilylation with bifunctional olefinic ligands to enable water solubility and bioconjugation. When uniformly dispersed in water, the SiNPs with an average particle size of about 114 nm showed remarkably high extinction coefficients, more than 2 × 1010 M–1 cm–1, from the visible to near-infrared spectral range. For the first OR-PAM imaging demonstration, the SiNPs were microinjected to live zebrafish embryonic cells. As the embryo developed, the distribution of the cells carrying the SiNPs can be clearly traced by the spread of the photoacoustic signals. Second, photoacoustic-fluorescence dual-modality imaging was demonstrated on live zebrafish larvae which were microinjected with a mixture of the SiNPs and dextran-rhodamine dyes. Due to the large difference in size, the two agents exhibited very different biodistributions after being injected at the same instant. Third, immuno-photoacoustic imaging, as an autofluorescence-free alternative to the conventional immunofluorescence imaging, was demonstrated on SKOV-3 cancer cells immunostained with the anti-HER2-conjugated SiNPs. Lastly, advantages of the SiNPs compared to other photoacoustic contrast agents were characterized and discussed.